The development of the electrophysiological and morphological characteristics of alpha spinal motoneurons from postnatal day 0 to 14 will be investigated using a hemisected and an intact, in vitro spinal cord preparation. The electrophysiological properties to be investigated include the passive membrane characteristics, in particular input resistance and space and time constants. Active membrane properties will be determined following dorsal and ventral root stimulation and intracellular current injection. Action potential (amplitude and duration), afterpotential (amplitude and duration), and repetitive firing characteristics will be studied. The ionic basis for motoneuron firing will be determined using current clamp methodology. Substitution of ions in the bathing medium and specific pharmacological agents will allow for the separation of the several ionic species known to underlie firing in mammalian nerve cells. The characteristics of synaptic potentials during postnatal development will also be studied with emphasis on the properties of the Ia EPSP and the development of those synaptic inputs mediating 'direct' facilitation and inhibition as well as recurrent facilitation and inhibition. The progressive differentiation of electrophysiological characteristics which accompany motoneuronal innervation of fast and slow twitch muscles will also be characterized. Changes in, and the differences between the morphology of HRP-filled motoneurons identified as innervating slow or fast muscles will be investigated in detail. This will include the pattern and orientation of dendritic and axonal arbors using computer analysis and reconstruction. Somatic size and the number and distribution of primary dendrites of other HRP-filled motoneurons will also be determined for animals from birth to 14 days.